1. Field of the Invention
The present invention relates to a technique for X-ray CT imaging using an X-ray, and particularly to a technique of improving the resolution of a CT image.
2. Description of the Background Art
In the medical field, an X-ray imaging apparatus allowing various imaging modes to be performed by itself alone is used. For example, in the dental field, Japanese Patent Application Laid-Open No. 9-122118 (1997) discloses a multi-purpose type X-ray imaging apparatus which provides both a panoramic imaging mode and a CT imaging mode. In the panoramic imaging mode, a curved dental arch is imaged as an image developed in a plane. In the CT imaging mode, a tomographic image of a region of interest in a living organ is acquired.
In the X-ray imaging apparatus disclosed in Japanese Patent Application Laid-Open No. 9-122118 (1997), an X-ray source and an X-ray image detection means are revolved while being opposed to each other with an object interposed therebetween, and a plane including the orbit of the revolution of the X-ray source and X-ray image detection means is defined as a plane of revolution. A fan-shaped X-ray beam is radiated while selectively switching an opening portion (slit) provided at the front of the X-ray generator between a slit for cross-sectional tomographic imaging (CT) and a slit for curved-plane tomographic imaging (panorama). The slit for cross-sectional tomographic imaging is elongated in a direction parallel to the plane of revolution, and the slit for curved-plane tomographic imaging is elongated in a direction perpendicular to the plane of revolution of a revolving arm.
Japanese Patent Application Laid-Open No. 9-122118 (1997) also discloses rotating the whole of the X-ray generator and the whole of an X-ray imager around an emission central axis connecting the X-ray generation part to the X-ray detection part, to selectively switch the arrangement of the X-ray generator and the X-ray imager between an arrangement for cross-sectional tomographic imaging and an arrangement for the panoramic imaging.
Additionally, several techniques have been proposed for improving the resolution of an image by improving a method of forming an X-ray beam (for example, Japanese Patent Application Laid-Open No. 2001-54515, Japanese Patent Application Laid-Open No. 2006-149493, and Japanese Patent Application Laid-Open No. 57-200132 (1982)).
More specifically, Japanese Patent Application Laid-Open No. 2001-54515 discloses CT imaging using a cone-beam X-ray in which a part of a cone-beam X-ray having a large effective focal spot is blocked by a leaf made of an X-ray blocking material.
Japanese Patent Application Laid-Open No. 2006-149493 discloses X-ray breast imaging in which an X-ray is partially blocked by a displaceable blocking plate so that X-ray emission can be performed using a part of the X-ray having a certain degree or more of intension and having as small a focal spot size as possible.
Japanese Patent Application Laid-Open No. 57-200132 (1982) discloses CT imaging in which an X-ray is generated at a plurality of different spots on a target surface of an X-ray tube at the anode side.
While, several techniques have been proposed for improving the resolution by reducing a focal spot size, a technique for improving an image resolving power in a multi-purpose type X-ray imaging apparatus which allows various types of X-ray imaging having different X-ray radiation ranges is not yet known.
Particularly, in the field of an X-ray imaging apparatus for dental use, there has been a demand that an X-ray beam having the optimum focal spot size for each of different modes be set in a multi-purpose type X-ray imaging apparatus allowing an imaging mode to be switched between a panoramic imaging mode, an X-ray CT imaging mode, and the like.
The present invention is made in view of the problem described above, and an object of the present invention is to provide a technique of improving the resolution of an X-ray image in an X-ray imaging apparatus which allows various types of X-ray imaging having different X-ray radiation ranges.
In the above-described multi-purpose type X-ray imaging apparatus, and particularly in the X-ray imaging apparatus for dental use, for example, in the panoramic imaging mode, a so-called narrow-type X-ray beam is used in which the spread of an X-ray beam radiated to an object for imaging is much smaller when compared with an X-ray CT imaging mode. In the X-ray CT imaging, on the other hand, it is necessary to use an X-ray beam whose spread in a horizontal direction corresponds to the size of an imaging area, in order to cover the imaging area.
In the panoramic imaging, a better image quality is obtained by making a focal spot size in the horizontal direction smaller. However, in a conventional apparatus, an X-ray is blocked from both sides thereof in order to adjust the spread of a radiated X-ray beam in accordance with switching between the X-ray CT imaging mode and the panoramic imaging mode. In a case where an X-ray radiation range is narrowed in this manner, the apparent size of a focal spot is hardly changed. Therefore, it has been difficult to improve the image quality by, for example, increasing the sharpness of an X-ray image or reducing blurring (so-called defocusing or reflection blurring), because when imaging is performed using a narrowly-spread X-ray beam, an X-ray passing through a certain spot on an X-ray emission path is largely dispersed on an X-ray detection surface so that the resolution is lowered.
In a case of an X-ray imaging apparatus providing only X-ray imaging using an X-ray beam narrowly spread in the horizontal direction, such as the panoramic imaging, the imaging may be performed using an X-ray beam having a small focal spot size. However, a certain degree of focal spot size is required in order to ensure an emission region for the CT imaging mode using an X-ray beam largely spread in the horizontal direction. Accordingly, in an X-ray imaging apparatus providing both of X-ray imaging using an X-ray beam widely spread in the horizontal direction and X-ray imaging using an X-ray beam narrowly spread in the horizontal direction, an X-ray imaging apparatus has been demanded which can meet the requirement that the sharpness of an X-ray image be further increased in X-ray imaging using an X-ray beam narrowly spread in the horizontal direction.
Particularly, in a case when a cone-beam is used for the CT imaging, the panoramic imaging can be also performed using a narrow X-ray beam obtained by restricting the spread of the cone-beam in the horizontal direction. Here, an X-ray imaging apparatus which can increase the sharpness of an X-ray image obtained by panoramic imaging also has been in demand.
As for the CT imaging mode, too, in a CT imaging apparatus providing both of a large or wide Field of View (FOV) imaging mode which is a CT imaging mode using an X-ray beam widely spread in the horizontal direction and a small or narrow FOV imaging mode which is a CT imaging mode using an X-ray beam narrowly spread in the horizontal direction, an X-ray imaging apparatus has been demanded which responds to a problem that the sharpness of an X-ray image should be increased in the small FOV imaging mode.
Moreover, an X-ray imaging apparatus has been demanded which can, in addition to increasing the sharpness of the X-ray image in each imaging mode, improve the image quality with an enhanced accuracy and a low cost, and moreover suppress the amount of X-ray exposure of an object.
In the X-ray CT imaging mode, the degree of spread of an X-ray beam used is sometimes different between a case where an imaging region is relatively large or wide and a case where an imaging region is relative small or narrow. In such a case, similarly to the above-described case, it is demanded to improve the image quality when imaging is performed by using a narrowly spread X-ray beam.
Japanese Patent Application Laid-Open No. 9-122118 (1997) gives no consideration to a focal spot size corresponding to each imaging type, and merely switches an X-ray radiation field between a radiation field elongated in a direction parallel to the plane of revolution and a radiation field elongated in the direction perpendicular to the plane of revolution in accordance with whether the imaging is a cross-sectional tomographic imaging or the panoramic imaging. Thus, it is difficult to optimally increase the sharpness of an X-ray image in accordance with the type of imaging.
In the X-ray imaging apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-54515, only a single type of CT imaging mode is provided, and the mode itself cannot be switched.
In the X-ray imaging apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-149493, an X-ray radiation range can be adjusted, but only a single type of X-ray imaging mode such as X-ray breast imaging is provided, and the mode itself cannot be switched.
The X-ray imaging apparatus disclosed in Japanese Patent Application Laid-Open No. 57-200132 (1982) does not aim at changing the focal spot size of an X-ray beam, but aims at obtaining an X-ray image of a plurality of slice planes by using X-rays from a plurality of focal points. It is impossible that the sharpness of an X-ray image is increased in an imaging mode using an X-ray beam spread more narrowly.
Any of the Patent Documents mentioned above does not contain the idea of making a control such that the angle formed between the central axis of an X-ray beam emitted from an X-ray generation part and a straight line connecting the cathode and the anode of an X-ray generation source is changed by switching an X-ray imaging mode in accordance with a reduction in an X-ray beam radiation range. Therefore, it is impossible that the sharpness of an X-ray image is increased in an imaging mode using an X-ray beam spread more narrowly. Additionally, it is impossible to adjust the focal spot size in accordance with a mode and to increase the sharpness of an X-ray image with a simple structure and a low cost. Furthermore, it is impossible to allow the CT imaging to be performed in imaging regions having different sizes and to adjust the focal spot size, that is, to increase the sharpness of an X-ray image, in accordance with the size of the imaging region.